1. Field of the Invention
This invention relates to a method for isolating and purifying metal-containing solutions, and more specifically the invention relates to a method for enhancing demulsification and phase disengagement in metal extraction systems using magnets.
2. Background of the Invention
Selectively obtaining a compound or element from a mixture, liquid, solid, or gas, can be done in innumerable ways. One way is through solvent extraction. In solvent extraction, the chemical mixture containing a desired moiety is dissolved in a first solvent. The solution containing the moiety is then mixed with an immiscible second solvent that has a higher affinity for the desired moiety than the first solvent. After a thorough mixing of the immiscible solvents, the mixture settles, and the solvents separate over time. During the mixing and settling phases of a solvent extraction, the desired moiety will migrate between solvents because of the moiety's greater affinity for one of the solvents. Typically, the desired moiety should reside mostly or entirely in the second solvent after separation.
Frequently, a mixture containing a desired moiety is dissolved into an aqueous solution. The aqueous solution is then mixed with an organic solvent that is immiscible with water. Typically, the organic solvent will have a higher affinity for the desired moiety than the original aqueous solution. After the original aqueous solution and immiscible organic solvent are thoroughly mixed and then allowed to settle, the two solvents return to separate aqueous and organic phases based on differences in density of the aqueous and organic phases. After the desired moiety transfers to the organic phase, the organic phase is then called the “loaded organic phase.”
Many commercial applications of solvent extraction reuse the organic solvent. These commercial applications strip the loaded organic phase of the desired moiety, using a clean aqueous phase devoid of undesired impurities. The stripped organic phase is then recycled for use in later extractions.
Solvent extraction, sometimes called liquid-liquid extraction, is commonly used in a wide array of applications, from the small scale (e.g., use of a single separatory funnel in a laboratory) to the large scale (e.g., separation of crude oil). In industry, solvent extraction is commonly used to separate desired metal ions after dissolving ore in mining operations. For example, solvent extraction is commonly used to obtain rare earth metals, platinum group metals, base metals, and nuclear materials. As the world's supply of high grade ores containing these materials decreases, solvent extraction becomes more important because solvent extraction is more effective in retrieving desired materials from low grade ores than other techniques.
In industrial applications of solvent extraction, the efficiency of the entire process is often hindered by demulsification issues, i.e., the slow or incomplete separation of the two immiscible solvents used in the solvent extraction process. In such circumstances, two conventional solutions are utilized: (1) spinning the mixture of emulsified immiscible solvents in a centrifugal contactor and (2) demulsification-aiding additives.
Solvent extractions hindered by slow demulsification often utilize centrifugal contactors to spin the mixture of immiscible solvents in order to separate the solvents based on the difference in the two solvents' densities. These centrifugal contactors are effective in demulsifying extraction mixtures that would otherwise very slowly or never completely demulsify without intervention.
Centrifugal contactors are expensive to incorporate into existing processing schemes and refineries, mechanically complex, and difficult to maintain. Large scale solvent extractions hindered by demulsification inefficiencies require many centrifugal contactors, causing large initial and ongoing costs. Also, centrifugal contactors are less effective when separating phases with similar densities; a common occurrence in solvent extraction systems where the organic phase becomes heavy as it is loaded with metal ions.
Other methods for increasing the efficiency of solvent demulsification include the use of chemical additives. However, chemical additives also have drawbacks. First, purchasing chemical additives on a large scale and on an ongoing basis is expensive. Also, to ultimately obtain a purified desired moiety, any demulsification aids must be removed to obtain that purified final product. Further, chemical additives must be removed downstream from extraction processes in order to recycle solvents and to prevent runoff into the environment. Removing additives downstream from an extraction process therefore requires extra steps and additional costs.
Aside from using chemical additives, physically inert solid phase additives have been employed. Specifically, researchers have added magnetic nanoparticles to emulsifications and then subjected that mixture to magnetic fields in attempts to change the magnetic susceptibility of one component of the emulsion, thereby enhancing its separation tendencies. However, as with the use of chemical additives, these additions recreate secondary waste streams which must be addressed. For example, the magnetic particles need to be removed downstream during final polishing of the target moiety.
Furthermore, the addition of relatively inert aggregate material stymies the fluidity of the mixture, particularly in pumping protocols. Therefore, such magnetic particles, fluidized beds, etc., are contraindicated when continuous, pumpable, all liquid processes are sought.
Thus, a need exists in the art for a method and system of accelerating the demulsification of solvents in solvent extractions that are hindered by slow or indefinite demulsification. Ideally, the method and system would not require modification of the extraction infrastructure or the removal of unwanted impurities downstream (i.e., a secondary waste stream).